fix: dataset after insert when db relation does not exist

[dpgaspar]

SUMMARY

Fixes a regression when loading examples would trigger dataset after_insert SQLAlchemy event with dataset.database as None, previously these triggers for creating new permissions were a noop and a second trigger would occur with the expected database relation. Currently the second event does not occur, so this PR will handle dataset.database as None by fetching the database model itself.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

Before: after loading examples, not all datasource access permissions would be created (the number could vary)

After: All permissions are created has expected (23 datasets create 23 datasource access permissions)

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #21492 (22175e0) into master (4fcc1d9) will decrease coverage by 1.49%.
The diff coverage is 83.13%.

❗ Current head 22175e0 differs from pull request most recent head e255133. Consider uploading reports for the commit e255133 to get more accurate results

@@            Coverage Diff             @@
##           master   #21492      +/-   ##
==========================================
- Coverage   66.67%   65.17%   -1.50%     
==========================================
  Files        1791     1793       +2     
  Lines       68459    68495      +36     
  Branches     7268     7277       +9     
==========================================
- Hits        45643    44644     -999     
- Misses      20955    21989    +1034     
- Partials     1861     1862       +1     

Flag	Coverage Δ
hive	?
mysql	78.20% <92.45%> (+0.01%)	⬆️
postgres	78.27% <92.45%> (+0.01%)	⬆️
presto	?
python	78.33% <92.45%> (-3.07%)	⬇️
sqlite	?
unit	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...et-ui-chart-controls/src/shared-controls/index.tsx	59.61% <0.00%> (-1.17%)	⬇️
.../superset-ui-core/src/query/normalizeTimeColumn.ts	100.00% <ø> (ø)
.../plugin-chart-echarts/src/Timeseries/buildQuery.ts	66.66% <ø> (ø)
...gin-chart-echarts/src/Timeseries/transformProps.ts	52.38% <0.00%> (-1.20%)	⬇️
...ontend/src/SqlLab/components/QuerySearch/index.tsx	73.07% <ø> (ø)
...rset-frontend/src/components/AsyncSelect/index.jsx	90.00% <ø> (ø)
...c/components/DeprecatedSelect/DeprecatedSelect.tsx	53.33% <ø> (ø)
...d/src/components/DeprecatedSelect/NativeSelect.tsx	57.14% <ø> (ø)
.../src/components/DeprecatedSelect/OnPasteSelect.jsx	78.37% <ø> (ø)
...precatedSelect/WindowedSelect/WindowedMenuList.tsx	4.76% <ø> (ø)
... and 83 more

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